This invention relates generally to current transfer brushes for electric machines, and more particularly, to a sliding stack arrangement of graphite foils, each of which is provided with a layer of electrically conducting material.
Current transfer brushes which are in general use for transferring electric current from a supply circuit to a rotating member of an electric machine are generally formed of electrographite, which may consist of natural graphite or a mixture of a metal and graphite. Such graphite brushes are known to possess the characteristics of high electrical conductivity and low friction between the brush and the rotating member of the electric machine, illustratively a slip ring or commutator. The principal operational characteristics of such brushes are determined by the coefficient of friction .mu., which is a function of the relative velocity between the brush and the rotating machine part, and by the voltage drop .DELTA.V, wich is a function of the density of the current which is conducted by the brush. The values of both such quantities are directly related to a surface skin which is formed on the rotating contact member of the electrical machine. Such a surface skin is called film or patina, and is composed of the materials of the brush and the rotating contact member which are abraded during operation. The thickness and composition of the surface skin is related to a variety of factors, including the material composition of the graphite of the brush, the composition of the rotating contact member, the current density, the relative velocity between the brush and the rotating member, and the temperature of the contact member. In addition, the nature of the surface skin is affected by the contact pressure between the brush and rotating member, and a variety of variable factors including atmospheric conditions, air humidity, and the presence of chemical gases and vapors.
One current transfer brush which may conduct a high density of current with low contact pressure and abrasion is disclosed in German Offenlegungsschrift No. 28 17 371. The sliding member of the brush described therein contains a multiplicity of graphite foils which are composed of highly graphitized graphite and combined in a stack. The graphite material used in the brush contains a high percentage of crystallized graphite. Each such foil may be provided with a layer of an electrically conducting material on at least one side, so as to reduce the resistance of the brush and the corresponding voltage drop. In operation, the brush is disposed so that the edges of the foils in the stack contact the rotating member of the electrical machine.
It is a problem with the stack-type current transfer brush described in the German patent reference that the contact resistance disadvantageously increases with the relative velocity of the rotating machine member. Such an increase in resistance results from the fact that the slip rings or commutators used in the electric machines are not perfectly round. Thus, even minute eccentricities in the slip rings or commutators will cause the relatively inflexible graphite foils to separate from the slip rings or commutators. A separation of even a few micrometers leads to the so-called "brush fire" which causes an increase in the electrical contact resistance and in the rate of material removal. Such brush fire is generally difficult to surpress.
One prior art attempt to improve stack-type brushes is disclosed in German Offenlegungsschrift No. 28 17 402. This reference teaches a current transfer brush which is more flexible than that described in German Offenlegungsschrift No. 28 17 371 by including mats or felts of highly graphitized graphite fibers in the stack arrangement. However, although the brush containing the mats or felts is flexible, such a flexible brush nevertheless exhibits strong brush fire and high rate of material loss during operation.
It is, therefore, an object of this invention to improve current transfer brushes of the type which utilize graphite foils in a stack so as to reduce the occurrence of brush fire without increasing the rate of material removal.